ClickPass (YC 07, http://clickpass.com/) was doing this, but their site says not to develop against them atm.
JanRain does it, but costs money (http://www.janrain.com/products/engage/get-janrain-engage).
Any open source implementations or other alternatives?
It seems like one can get away with the free plan pretty well. But the real issue is that now if either you or they have downtime, you have downtime. I assume they have some good HA practices, but one of the times that I was working on debugging the issue, I was getting 500s.
An open source version, especially one that could be easily used with any language, would be awesome.
It's possible that the best way to do that would be to essentially clone Janrain open-source-ly and write libraries for that clone. That way most of the work is being done in whatever language it's implemented in, and then to make it work in a new language, you'd just write a simple library integrating with its API, rather than rewriting the whole thing.
The advantage is that you'd host it yourself, and so you can take steps to ensure that it stays up. (And at the very least, it would be hosted in the same datacenter, so unless your hosted version of JanrainClone crashes, its downtime would mostly coincide with your own downtime.)
Their free version of Engage is sufficient for basic authentication. As mentioned already, your authentication system will then rely on your site, Janrain's site, and also the 3rd party site that performs the actual authentication. If you just want to quickly get going, you can use their free version of Engage with the above caveats.
If you want to not rely upon JanRain, then you can use one of the OpenID libraries out there. Integration is "relatively" simple, it should take 1 day in most cases.
However, you may want more than OpenID (Hello, Facebook Connect and Twitter's OAuth). If that is the case, then you will have to add support for those sites as well.
It would be nice if there is an open source version of Engage, but there doesn't appear to me from my searching.
I moved away from them because I wanted to use it in a Facebook application to generate user accounts on the site but this wasn't possible without upgrading further. Also, 99% of my users came from Facebook or Twitter and I just didn't need the other options.
i think this is what stackoverflow uses